The present invention relates to a crosslinking agent for use in the production of a highly water-absorbing polymer crosslinked in an aqueous medium and comprising a polymerizable compound having a carbon-carbon double bond or a salt thereof.
Most of highly water-absorbing polymers comprising polymerizable compounds having polymerizable double bonds (e.g. carbon-carbon double bonds) or salts thereof are based on acrylate salt polymers and produced mainly by polymerization in an aqueous solution. It is proposed to use a wide variety of materials having a reactive double bond, such as acrylate esters, acrylic amides and allyl ethers, as crosslinking agents for crosslinking the highly water-absorbing polymers, It is reported that particularly when allyl compounds are used as the crosslinking agent, the polymers excellent particularly in water absorptivity can be obtained. Further, the reverse-phase suspension polymerization mathod polymerizing a monomer and a crosslinking agent dissolved in water suspended in an organic solvent is also industrially carried out, and this reverse-phase suspension polymerization method can also be regarded as the polymerization in an aqueous medium.
For example, it is reported in J. Polym. Sci. A: Polym. Chem. 35, 799 (1977) that polymers obtained by using polyethylene glycol diallyl ether as the crosslinking agent is superior in water absorptivity to the polymers obtained by using an acrylic crosslinking agent. However, the performance of the polyethylene glycol diallyl ether as the crosslinking agent is not satisfactory.
A method of neutralizing polymers obtained by polymerizing acrylic acid is known. JP-A-3-174414, which uses this method, discloses use of tetra-allyloxy ethane as a specific allyl compound. However, this compound suffers from problems such as deficient heat resistance, poor solubility in an aqueous solution of a monomer, and insufficient resistance to hydrolysis. Thus there is demand for development of crosslinking agents having higher performance.
Further, JP-A-4-246403 discloses use of triallyl amine, trially cyanurate, triallyl isocyanurate and triallyl phosphate. However, these crosslinking agents generally have problems such as deficient heat resistance, adverse effects on polymerization reaction, poor solubility in an aqueous solution of a monomer, and insufficient resistance to hydrolysis, and thus none of these crosslinking agents are practical.
Generally in the aqueous solution polymerization, an aqueous solution of an acrylic acid monomer is neutralized at a degree of about 75% with e.g. an aqueous solution of sodium hydroxide, then a crosslinking agent is mixed therewith, the monomer is polymerized by a polymerization initiator, and the formed solid is cut into pieces of suitable size and then dried (hereinafter, this method is referred to as xe2x80x9cmethod of polymerization after neutralizationxe2x80x9d). On the other hand, the crosslinking agent is dissolved in an aqueous solution of acrylic acid, and the solid formed by polymerization is cut and neutralized (hereinafter, this method is referred to as xe2x80x9cmethod of polymerization before neutralizationxe2x80x9d). This method is disadvantageous in production efficiency and uneven neutralization of the product, in comparison with the neutralization in solution form.
The object of the present invention is to provide a novel allyl type crosslinking agent soluble in an aqueous solution of a monomer (e.g., an aqueous solution of an acrylate salt) and free of the problems possessed by the prior arts, in order to produce a highly water-absorbing polymer having water-absorptivity required at practical levels.
The present invention provides a crosslinking agent for use in the production of a highly water-absorbing polymer comprising a polymerizable compound having a carbon-carbon double bond or a salt thereof, comprising a polyallyl ether compound having at least two allyl groups and at least one hydroxyl group derived from a glycidyl group.